Caption: DNA loop. Theoretical molecular model of a loop structure for DNA in its chromatin form. DNA (deoxyribonucleic acid) is a helical molecule (here the helix itself forms a circular loop) made up of two interlocking sugar-phosphate strands (wavy pink lines) bound together by their nucleotide bases. Two loops of chromatin are seen here, with around 16 'twists' of the helix per loop. Chromatin, found in cell nuclei, stores DNA in a condensed coiled form, as seen here. This model shows 171 base pairs. The atoms (tubes) are: carbon (light blue), oxygen (red), phosphorus (pink), and nitrogen (blue). Hydrogen atoms are not shown.